Method of dissolving a plug from a bimetallic metal core and etching bath



y 9, 1966 K. P. BELLINGER 7 3,261,733

METHOD OF DISSQLVING A PLUG FROM A BIMETALLIC METAL CORE AND ETCHINGBATH Filed July 2, 1963 X y \WM 1 M 1 INVENTOR KENNETH P. BELLINGERa/a/M A T TOR/VE) United States Patent 3,261,733 METHGD 0F DISSOLVING APLUG FROM A El- METALLIC METAL CORE AND ETCHING BATH Kenneth P.Bellingcr, Ellington, Conn, assignor to Conversion Chemical Corporation,Rockville, onn., a corporation of Connecticut Filed July 2, 1963, Ser.No. 292,325 6 Claims. (Cl. 156-3) The present invention relates to metalfabrication and more particularly to a novel bath and method forremoving cores of iron, molybdenum and copper from initial assemblieswith other metals.

This application is a continuation-in-part of copending applicationSerial Number 254,426, filed January 28, 1963, now abandoned.

In the fabrication of various metals such as titanium, zirconium,tungsten, aluminum and their alloys, it is often desirable to employ acore of another metal about which the primary metal is formed. Forexample, in forming nuclear reactor cores, plugs of iron or copper maybe employed to define the desired apertures and titanium or zirconium isformed therearound. Subsequently, the core is dissolved in a bath ofnitric acid. In forming filaments for light bulbs, tungsten wire iswound around an iron or molybdenum core and set; thereafter, the core isdissolved in a bath of nitric acid. Generally, such core-removingoperations are characterized by a very high degree of exothermicactivity, extremely rapid metal removal and copious fuming. The highlyexothermic and rapid reaction has created serious problems in control ofthe operation, and fumes evolved by the operation have presented mostsignificant problems from industrial hygiene and anti-pollutionstandpoints. Various scrubbers and treatments have been proposed andutilized at great expense and without complete efiicacy in an effort tocope with the problem.

The term core as used herein refers to inserts, mandrels and variousshapes of dissolvable metals selected from the group consisting of iron,copper and molybdenum which are readily dissolved by nitric acid baths.Generally, low carbon iron or cold-rolled low carbon steel of low alloycontent are the preferred ferrous materials to avoid contamination byalloying elements. Similarly, copper and molybdenum should 'be of lowalloy content.

It is an object of the present invention to provide a novel bath forremoving metal cores from initial assemblies with other metals formedthereabout and substantially inert to the bath which bath ischaracterized by rapid but relatively controlled action andsubstantially freedom from fuming.

Another object is to provide a method for removing metal cores frominitial assemblies with other metals which is rapid, relativelycontrolled and substantially free from fuming.

Other objects and advantages will be readily apparent from the followingdetailed specification and claims and the attached drawing wherein:

FIGURE 1 is a diagrammatic illustration of apparatus in which a processembodying the present invention is being conducted.

It has now been found that the foregoing and related objects may bereadily attained by forming about a dissolvable core of a metal selectedfrom the group consisting of iron, molybdenum and copper a primary metalto produce an initial assembly; and thereafter immersing the initialassembly in a core-removing bath for a period of time sufiicient todissolve the core, the core-removing bath comprising an aqueous solutionconsisting essentially of 15.0 to 42.0 percent by weight nitric acid,2.0 to 28.0 percent by weight of a reaction-controlling agent selectedfrom the group consisting of urea and melamine, and 40.0 to 80.0 percentby weight water. In operation, the

bath should develop a foam blanket for optimum effectiveness ineliminating nitric oxide fumes, but excessive frothing should be avoidedto maintain effective core-dissolving action as by a cooling grid abovethe bath or by ultrasonic energy. This method has significant advantagesin the forming of tungsten, zirconium, titanium and aluminum since iteffects the speedy removal of the core in a relatively controllablereaction while being substantially innocuous to the formed primary metalthereabout.

The molar ratio of reaction-controlling agent to nitric acid is about0.1 to 0.75:1, and preferably about 0.25 to 0.60:1 for iron andmolybdenum cores and about 0.15 to 0.45 :1 for copper cores. When thenitric acid is in the range of about 35.0 to 42.0 percent by weight ofthe bath and/or when the iron or molybdenum core metal present in thebath is suflicient to expend substantially the nitric acid, it isgenerally necessary to use a molar ratio of about 0.3 to 0.621 toeliminate turning and maintain the reaction under control. When treatingcopper cores, it is generally necessary to use lower weight percentagesand molar ratios of reaction-controlling agent since it appears toproduce or favor the formation of a barrier coating on the copper corewhich limits the attack of the nitric acid. Use of the higher molarratios of reaction-controlling agents may produce a significant increasein viscosity and/ or the development of a pasty condition due to theformation of a precipitate requiring cont-r01 of the frothing and/ orincrease in temperature.

The bath temperature should be maintained in the range of about to 210Fahrenheit, and most desirably about to 200 Fahrenheit. When more thanabout 10 percent by Weight of reaction-controlling agent is used, it isgenerally necessary to maintain a temperature of about 180 to 200Fahrenheit to dissolve the agent and to reduce excessive viscosity inthe bath. Temperatures of about 170 to 200 Fahrenheit are desirable formaintaining optimum effectiveness of the bath in removing copper cores.Although the bath generates heat during treatment, generally it isnecessary to heat the bath initially to obtain full solution of ordispersion of the higher weight percentages of urea. Particularly whenthe bath volume is small or when continuing use is being conducted withiron or molybdenum cores, cooling coils may be desirably provided toprevent the bath from overheating due to the exothermic reaction andfrom possibly reaching the boiling point or from developing excessivefroth.

The method of the present invention is rapid and highly exothermic butcan provide greater control than a conventional nitric acid bath whileeliminating substantially the evolution of nitric oxide. Since thereaction is controlled, nitric acid spray is reduced, and the hazards tothe operator are minimized. Removal of as much as onethird of an ironcore Within a minute in assemblies presenting relatively large surfacearea is possible with the preferred bath composition operating at thepreferred temperature range. Thus, rack treatment of assemblies ispossible with the time of immersion required being readily determinableby preliminarily treating a test assembly and visually determining thetime required for complete removal of the iron core in the remainingassemblies. Moreover, it has been found that the nitric oxide fuming maybe reduced from in excess of 300,000 parts per million to 300 parts permillion, and even less. In this manner, high rates of production may bemaintained at elevated bath temperatures without expensivefume-scrubbing installations.

The preferred baths of the present invention for iron and molybdenumcores consist essentially of about 28.0 to 38.0 percent by weight nitricacid, 47.0 to 54.0 percent by weight water and 8.0 to 20.0 percent byweight of the reaction-controlling agent with a molar ratio ofreactioncontrolling agent to nitric acid of about 0.2 to 0.6: 1, and

most desirably of about 0.35 to 0.55: 1. For copper cores,

the bath preferably contains only about 4.0 to10.0 percent ofreaction-controlling agent with a molar ratio of about 0.15 to 0.45:1.

Al-thohgh melemthe y he usedelehe Q p e the tially complete eliminationin the treatment tank is de- IOWEI' molar ratios and Wltl'l urea Incombination, urea sirable to ensure efiicacy of the bath peration andon. has e found far t e Preferable because of P h trol and to minimizehealth hazards resulting from poseperetlhg ehareeterlstles, e lowermolecule" welght sible defects in the exhaust or scrubbing systems. Withand greater eohthlhty t0 PYOVlde the more etteetlve hlghel" thecore-dissolving baths of the present invention, scrubrnolar ratios. 0bing the fume in water alone or water with some alkali Although the ureay he added Increments during is sufficient to remove the nitric acidfumes and any trace the operat n Of the bath, thls tq Preludes? leesamounts of other noxious gases such as nitrogen dioxide ettectlve bathand P e Substantial hazards slhee a since the predominant nitric oxideis apparently converted relatlvely t p reactlon takes Place When urea 18added to nitrogen within the bath and its foam blanket, analyses t a hotnltfle e 1S0hltl0h- Le amounts of urea indicating the presence ofnitrogen and carbon dioxide. y he used With lherelhehtal eddltlen offurther amounts The initial assembly should be completely immersed withsome loss in effectlveness when a plurality of cycles i hi h b h d bCovered b more h f a d Will fun in a glveh hath e the urea is addedSlowly desirably more than six, inches of bath. In practice, the betweenye The h 15 Preferably h e by deeper is the immersion, the moreefficient is the fume first dissolving the urea in hot water because of1ts l1rn1ted removal h bli can b k d or i di id ll ylowered into thebath.

The method of the P e lhvehtleh 1S rapld and hlghty Referring now to theattached drawing, therein diaeXethermle hh PYOVlde g e e t than a lgrammatically shown is a process embodying the present ventional nitricac1d bath while eliminating substantlally invention The Work tank 2Contains h b h 4 i hi h the evolution of nrtrieoxlde fumes. The baths ofthe is immersed the initial assembly ggnerally designated b Presentlhvehtloh hlhetloh ethelehtly even when the the numeral 6 and having acore 8 of a dissolvable metal face of the metal has an e t Settle andappear to with the primary metal 10 formed thereabout. As illushanee taetlOIl 0f the hlttle held on m eores- Y trated, the assembly 6 isdisposed well below the surface achlehhg cohtrol of the rate of thereachohg the exces of the bath 4 which has a foam blanket 12 ofsubstantial sive frothing produced by gas bubbles within the bath depththereoven Any fumes and Spray l i fr can be reduced so that the bath canfunction more eifi- ,the bath 4 are entrapped in h hood 14 and i d bythe conduit 16 to the bottom of the scrubber :tank 18 so Although thetheehehlem t the Present lhvehtloh 15 that they are washed thereby priorto being discharged hot y o d, it 1s be heved that the urea coopto theatmosphere by the conduti 20. As hereinbefore erates with the nitricacid wrthm the above-described comstated, scrubber may use Water alone,Water i .positlenhl hmlts at temPeraturjcs above Fahrehhelt 'ing urea,or water containing alkali to neutralize nitric by modifying thereaction taklng place at the bath-core acid fumes. inte f The rate ofmetal attack r rses rapidly W Illustrative of the eifect of varying themolar ratio increase n temperature to about 210 Fahrenheit and (andWeight percentage) of urea in a bath containing the solubility of ureaincreases (or the viscosity of the 40 50.0 percent by volume f nitricacid B and mixture decreases) to permit increase in its beneficial ac-500 percent by volume of Water is Table In the tests tron. In contrastwith conventional baths Wl'llCll have reported in Table 1, ferrousSpecimens were immersed in a deep brown color after contact with thecore, the baths 200 of the bath in a 500 graduate f one hour, of ePresent lhvehhoh geherahy Tetalh a greehtsh hue 45 the bath having beenpreheated to the indicated temperaahd, In Place of a heavy frothgeherhhy have a blanket ture. The ferrous specimens in these tests areplural of foam over the top of the bath. This foam presumably ieces ofWire of 0.196 inch diameter and about 0.25 inch Presents a large surfacee of urea "h f'oam Wah cut on a diagonal. The wire was of 1013 alloynominally acting to increase the l1kel1hood of reaction with fumescomposbd f 14 percent b i h carbon, percent e p g from e y of T teamlayer 50 by weight manganese with the remainder substantially initiallyincreases in depth with increase 1n urea conceni ith ulf r andphosphorus impurities. The prestration and also appears to increase inviscosity to a point en e or ab n of brown nitri oxide fumes above theof relatively highly concentration after which the foam bath in thegraduate and weight loss are noted as evilayer seems to stabilize ordecrease in depth. dencing the efficacy of the baths of the invention.

TABLE 1 HNO H2O, Urea, Molar Tempera- Time for Main Re- Wt, PercentPercent Percent Ratio Steel, ture, action to Subside Filming Loss,

by wt by wt. by wt. Urea: Grams Degrees or Steel to Dis- Gram IINOa F.solve, Minutes 39. 2 60.1 0. 7 0. 02; 1 20 180 39. 2 60.1 0. 7 0. 02=1 4180 39. 0 59. s 1. 2 0. 04; 1 20 180 39. 0 59.8 1. 2 0. 04:1 4 180 33. 759.1 2. 2 0. 00; 1 20 180 38.7 59.1 2. 2 0. 05:1 4 180 37.8 57.3 4.40.131 20 180 do 15.3. 37. 8 57. 8 4. 4 0.13:1 4 180 10 (c0nsun1ed) Heavy3 mm, diminished to Total light or none. 36. 3 55. 3 8.4 0.25:1 20 1B040 }Heavy 5 min., diminished to 16.4. 36. 3 55. 3 8. 4 0.25:1 4 180 10tconsumedym. medium, then to light. {Total. 35.0 53.5 11.4 0. 35:1 20180 Medium, diminishing to light {17.3. 35. 0 53. 6 11. 4 0. 3511 4 180after 1 minute. Total 33. 6 51. 6 14. s 0. 50:1 20 Slight iorao seconds.17. 0. 33.6 51.6 1 1.8 0.50:1 4 190 10 (consumed). do Total 39.5 60.5 2012 (c0nsumcd) 1 Frothed out of graduate.

Illustrative of the efficacy of the present invention are the followingspecific examples:

Example 1 A bath was prepared by admixing 100 cc. of nitric acid (42Baum) and 100 cc. of water. A slug of low car- TABLE 2 HNOE, H20, Urea,Molar Tempera- Time for Main Re- Wt, Percent Percen Percent Ratio Steel,ture, action to Subside Fuming Loss,

by wt. y wt by wt Urea: Grams Degrees or Steel to Dis- Grams HNOa F.solve, Minutes 1 Froth spilled out of graduate.

In Table 3 are set forth the results of similar tests on copper using asspecimens pieces of #12 gauge wire to present a relatively high surfacearea.

bon steel (No. 1013, containing 0.14 percent by weight carbon and 0. 65percent by weight manganese) weighing 25.3 grams was immersed in thebath which had been TABLE 3 HNO3, H1O, Urea, Molar percent percentpercent Ratio, Cu, Temp, Fuming Wt. Loss,

by wt. by wt. by wt. Urea: G. F. Grams I-INO;

39. 2 60.1 0.7 0. 02:1 33 180 1 18.8 39.0 59. 8 1. 2 0. 04:1 33 180 118.3 38. 7 59.1 2. 2 0. 06:1 33 180 1 25.0 37. 8 57. 8 4. 4 0.13:1 33180 2 17. 9 37. 8 57.8 4. 4 0.13:1 11 180 Light, 5 min Total 36. 3 55. 38. 4 0. 25; 1 33 180 Med, 2 min. 15. 2 35.0 53. 6 11. 4 0. :1 33 180None.-. 3 7. 5 39. 5 60. 5 33 180 Heavy 29. 6 35. 0 53. 6 11. 4 0. 35:133 4 190 None... 3 9.0

1 Spilled out of graduate badly. 2 Spilled out to small amount.

3 Green crystals formed in graduate during reaction and copper becamecoated.

' Heat continuously applied.

Illustrative of the several variables of the bath of the presentinvention on activity are the data set forth in Table 4. Test specimensof SAE 1010 cold-rolled steel 1 inch x 1.5 inches x 0.030 inch andweighing 5 grams were suspended in aqueous baths of the indicatedcomposition at the temperatures and times indicated. Within thepreferred range of bath composition, even initial fuming during theshort period of immersion was eliminated. It will be noted that amountsof nitric acid or urea in excess of the aforedescribed compositionallimits greatly affected the activity.

place-d in a 500 cc. graduate and preheated to 180 Fahrenheit. Thereaction was accompanied by violent pumping and agitation with a frothreaching nearly to the top of the cylinder. Extremely copious brownfumes rose from the bath which had a dark brown color. After aboutfifteen minutes, the reaction subsided appreciably and after one hourthe weight loss was found to be 17.9 grams.

TABLE 4 HNOJ, H 0, Urea, Temp. Time, Weight percent percent percentIncrease, See. Loss, Fuming by wt by wt. by wt. F. percent 1 49. 5 24. 226. 3 160-180 Paste Paste. 1 42.3 35. 4 22. 3 160-180 13 8 Heavy. 1 42.335. 4 22. 3 180-200 16 14 Do. 1 44. 3 44.0 11.7 160-180 12 18 D0. 1 39.7 39. 5 20. 8 160-180 15 15 D0. 1 40.8 48. 5 10. 7 160-180 12 0 Do.

36. 8 43. 8 19. 4 160-180 20 12 Moderate. 35. 6 42. 5 21. 9 180-190 13Very Slight. 38. 2 51. 7 10. 1 160-180 10 14 Heavy. 38. 2 51. 7 10. 1180-200 18 18 Do. 34. 8 46. 9 18.3 160-180 20 21 None. 34. 8 46. 9 18. 3180-200 12 14 Do. 34. 6 41. 1 24. 3 180-190 60 12 Do. 32. 6 50. 3 17. 1160-180 12 13 D0. 32. 6 50. 3 17. 1 180-200 18 27 Do. 19.3 70. 5 10.2160-170 11 Very slight. 19. 3 70. 5 10. 2 180-190 60 13 None. 16. 8 78.8 4. 4 -170 30 6 Do. 16. 1 75. 5 8. 4 160-170 30 8 D0.

1 These formulations were very pasty even at elevated temperatures dueto a precipitate.

A similar test was conducted with a bath additionally containing 42.0grams of urea and heated to 190 Fahrenheit initially. In this instance,a foam blanket was generated which extended to about four inches fromthe top of the graduate. The bath retained a greenish hue throughout andonly very light fuming occurred for the first two minutes during thedevelopment of the foam blanket and thereafter there was no evidence offuming. The reaction subsided substantially after twenty-tfive minutesand, after one hour, the weight loss was determined at 16.1 grams.

Example 2 A bath was prepared by admixing 200 cc. of 42 Baum nitric acidand 200 cc. Water. A test specimen of SAE 1010 cold-rolled steel 1 inchx 4 inches x 0.030 inch and weighing 18 grams was immersed in theresultant bath at an initial temperature of 180 Fahrenheit. The specimenwas withdrawn after the bath had reached a temperature of 200Fahrenheit, a time period of about thirteen seconds. The test specimenwas weighed and determined to have incurred a 26 percent weight loss.During the treatment operation, there was extremely copious fuming.

To a bath of the same composition was added 50 grams of urea and asimilar test specimen was immersed therein at a temperature of 180Fahrenheit and withdrawn after the bath temperature had reached 200Fahrenheit, a time period of sixteen seconds. The test specimen wasweighed and found to have incurred a 23 percent weight loss. During thetreatment, there was but a slight evolution of fume.

Another bath was prepared by adding 100 grams urea, and the test wasrepeated. The time for the bath temperature to rise from 180 to 200Fahrenheit was twenty-eight seconds, and the weight loss of the specimenwas 29 percent. No fuming occurred during the treatment.

Example 3 A bath was prepared by admixing 500 cc. of 42 Baum nitricacid, 500 cc. water and 125 grams melamine. A test specimen ofcold-rolled steel weighing 18 grams was immersed in the bath at aninitial temperature of 180 Fahrenheit and withdrawn therefrom after thebath had reached a temperature of 200 Fahrenheit, a time period of aboutforty seconds, during which there was extensive fume evolution.

The test was repeated with a bath containing 250 grams of melamine withthe time period for the temperature to rise from 180 to 200 Fahrenheitbeing about fifty seconds. The treated specimen was weighed and found tohave undergone a weight loss of 17.5 percent. During the period ofimmersion, there was but a very slight amount of fuming.

Example 4 Tungsten wire was wound about a core of molybdenum of variablediameter tapering to a point to form an initial assembly. The resultingassembly was immersed in a bath containing 50.0 percent by volume 42Baum nitric acid, 50.0 percent by volume water and 250 grams urea perliter and operated at a temperature of about 160 to 180 Fahrenheit.

The molybdenum core was dissolved out with substantially no fumeevolution and the tungsten wire was not attacked.

Example 5 A nuclear reactor component was produced by forming Zircalloy(nominal composition1.5 percent tin, 0.15 percent iron, 0.10 percentchromium, 0.05 percent nickel, and zirconium) about low carbon steelcores to define apertures therein.

The component was then immersed in a bath essentially the same as thatin Example 4 and operated at a temperature of about 160 to 180Fahrenheit. The steel cores were dissolved out with substantially nofume evolution. l

a the reaction was less violent.

8 Example 6 A bath was prepared by admixing 100 cc. of nitric acid (42Baum) and 100 cc. of water. Apparatus was provided to collect fumesevolved from the bath and wash them in a water scrubber provided by anErlenmeyer flask. The bath was preheated to 180 Fahrenheit and a lowcarbon steel plug of 25.3 grams was placed therein. The reaction wasviolent and heavy brown fumes flowed from the bath and through thescrubber. After about twelve minutes, the reaction slowed appreciablyand after one hour, the Weight loss was found to be 17.9 grams.

A similar test was conducted using a bath additionally containing 22grams of urea. Although the bath frothed, Moderate fuming for the firstfive minutes diminishing to light was evident in the bath vessel, butlittle or no fumes were evolved from the water scrubber. After one hour,the weight loss was found to be 16.4 grams.

Example 7 A bath was formulated by admixing 75 cc. of nitric acid (42Baum), 75 cc. of water and 12.5 grams of urea. A specimen of molybdenumweighing 1.6 grams was completely dissolved in eighteen minutes in arapid but controlled reaction. In a similar test bath containing nourea, heavy brown fumes were evolved.

Example 8 A bath Was formulated containing 50 percent by volume nitricacid (42 Baum) and 50 percent by volume Water. To 300 cc. of thisformulation which had been preheated to 180 Fahrenheit was added piecesof #12 gauge copper wire. Copious brown fumes were evolved and thereaction was violent.

A similar test was conducted with a bath additionally containing 24grams of urea. Slight fuming occurred only initially. After one hour,37.8 grams of copper was dissolved; after continuing immersionovernight, 39.8 grams was dissolved.

Thus, it can be seen that the method and bath of the present inventionpermit speedy but controlled removal of core metals of an initialassembly. Bath activity in some instances may be enhanced and in otherinstances is not extensively aifected, but bath action is morecontrolled and fume evolution is substantially eliminated to reducehygiene and fume-disposal problems.

Having thus described the invention, I claim:

1. The method of forming metals substantially inert to concentratednitric acid comprising forming about a core of a dissolvable metalselected from the group consisting of iron, molybdenum and copper aprimary metal substan tially inert to nitric acid to produce an initialassembly; and immersing said initial assembly in a core-removing bathfor a period of time suificient to dissolve said core, said bath beingat a temperature of about 140 to 210 Fahrenheit and comprising anaqueous solution consisting essentially of about 15.0 to 42.0 percent byweight nitric acid, 2.0 to 28.0 percent by weight of areactioncontrolling agent selected from the group consisting of urea andmelamine, and 40.0 to 80.0 percent by weight water, said bath having amolar ratio of reaction-controlling agent to nitric acid of 0.1 to075210, the immersion of said initial assembly in said bath producing ablanket of foam on the top of said bath through which fumes must pass.

2. The method in accordance with claim 1 wherein said substantiallyinert metal is selected from the group consisting of titanium,Zirconium, tungsten and alloys thereof.

3. The method of forming metals substantially inert to nitric acidcomprising forming about a core of dissolvable metal selected from thegroup consisting of iron, molybdenum and copper a primary metalsubstantially inert to nitric acid to produce an initial assembly; andimmersing said initial assembly in a core-removing bath for a period oftime sufiicient to dissolve said core, said bath having a temperature ofabout to 200 Fahrenheit and comprising an aqueous solution consistingessentially of about 28.0 to 38.0 percent by weight nitric acid, 8.0 to20.0 percent by weight of a reaction-controlling agent selected from thegroup consisting of urea and melamine, and 47.0 to 54.0 percent byweight water, the molar ratio of reaction-controlling agent to nitricacid being selected from the group consisting of (A) 0.35 to 0.55:1.0when said dissolvable metal is selected from the group consisting ofiron and molybdenum; and (B) 0.15 to 0.45:1.0 when said dissolvablemetal is copper, the immersion of said initial assembly in said bathproducing a blanket of foam on the top of said bath through which fumesmust pass.

4. The method in accordance with claim 3 wherein said substantiallyinert metal is selected from the group consisting of titanium,zirconium, tungsten and alloys thereof.

5. The method in accordance with claim 1 wherein UNITED STATES PATENTS2,177,751 10/1939 Sikorski 15618 X 3,192,084 6/1965 Vaughen et al 1563ALEXANDER WYMAN, Primary Examiner.

JACOB STEINBERG, Examiner.

1. THE METHOD OF FORMING METALS SUBSTANTIALLY INERT TO CONCENTRATEDNITRIC ACID COMPRISING FORMING ABOUT A CORE OF A DISSOLVABLE METALSELECTED FROM THE GROUP CONSISTING OF IRON, MOLYBDENUM AND COPPER APRIMARY METAL SUBSTANTIALLY INERT TO NITRIC ACID TO PRODUCE AN INITIALASSEMBLY; AND IMMERSING SAID INITIAL ASSEMBLY IN A CORE-REMOVING BATHFOR A PERIOD OF TIME SUFFICIENT TO DISSOLVE SAID CORE, SAID BATH BEINGAT TEMPERATURE OF ABOUT 140 TO 210* FAHRENHEIT AND COMPRISING AN AQUEOUSSOLUTION CONSISTING ESSENTIALLY OF ABOUT 15.0 TO 42.0 PERCENT BY WEIGHTNITRIC ACID, 2.0 TO 28.0 PERCENT BY WEIGHT OF A REACTIONCONTROLLINGAGENT SELECTED FROM THE GROUP CONSISTING OF UREA AND MELAMINE, AND 40.0TO 80.0 PERCENT BY WEIGHT WATER, SAID BATH HAVING A MOLAR RATIO OFREACTION-CONTROL LING AGENT TO NITRIC ACID OF 0.1 TO 0.75:1.0, THEIMMERSION OF SAID INITIAL ASSEMBLY IN SAID BATH PRODUCING A BLANKET OFFOAM ON THE TOP OF SAID BATH THROUGH WHICH FUMES MUST PASS.